WO2022071052A1

WO2022071052A1 - Porous carbon fiber, gas separation composite membrane, and gas separation membrane module

Info

Publication number: WO2022071052A1
Application number: PCT/JP2021/034757
Authority: WO
Inventors: 近藤大; 三原崇晃
Original assignee: 東レ株式会社
Priority date: 2020-09-29
Filing date: 2021-09-22
Publication date: 2022-04-07
Also published as: JPWO2022071052A1

Abstract

The present invention addresses the problem of providing a porous carbon fiber that is suitable for a support body for various gas separation membranes, with which it is possible to suppress adhesion between the separation membranes, the porous carbon fiber being characterized in that the fiber diameter varies in the length direction, and the CV% calculated based on the variation of the fiber diameter is 1-10%, inclusive.

Description

Porous carbon fiber, composite membrane for gas separation and module for gas separation membrane

The present invention relates to a porous carbon fiber and a composite membrane for gas separation using the same.

Porous carbon materials have long been used as adsorbents and reaction fields carrying catalysts. Further, in recent years, porous carbon fibers having communication holes have been reported, and a fluid separation film (carbon film) using the communication holes or a composite film for gas separation in which a separation functional layer is formed on the porous carbon fibers is used. It is expected to be utilized (see, for example, Patent Document 1).

Since fluid separation by a separation membrane uses pressure difference, concentration difference, and mass difference as driving force, running cost and equipment cost are low, and the required volume is small, so energy saving and compactness are possible compared to other separation methods. It is attracting attention as a method. In actual use, in order to increase the membrane area per unit volume, it is common to bundle a plurality of separation membranes, store them in a vessel, and then use them as a module. At this time, if the separation membranes are excessively adhered to each other, the area where the fluid comes into contact with the separation functional layer is limited, and there is a problem that the membrane efficiency is lowered. Further, there is a problem that the potting agent is sucked up between the bundles of the separation membrane by the capillary phenomenon at the time of modularization, and the effective membrane area is reduced.

Therefore, as a means for creating an appropriate distance between the separation membranes, a method of wrapping a fibrous material around the outer circumference of the separation membrane and a method of producing a separation membrane in which the fiber diameter changes periodically in the longitudinal direction have been proposed (for example, a patent). See documents 2 and 3).

International Publication No. 2019/021963 Japanese Unexamined Patent Publication No. 2001-334131 Japanese Unexamined Patent Publication No. 6-20407

Although Patent Document 2 discloses an example of a hollow fiber membrane in which a fibrous material is spirally wound around the outer peripheral surface, in order to take such a form, the step of winding the fibrous material around the separation film increases. , There was a problem of increasing the cost. Furthermore, in a separation membrane with small surface irregularities, the fibrous material slips and it is difficult to wind it at a constant pitch, and the fibrous material shifts during modularization, which suppresses adhesion between the separation membranes. In some cases, it was not effective as a means of doing so.

Further, Patent Document 3 discloses an example of a separation membrane in which spinning conditions are controlled and the fiber diameter changes periodically in the longitudinal direction. However, when the separation target is a gas, the separation membrane has pores on the order of angstroms. This method has a problem that the gas separation membrane is liable to have spots and the membrane performance is deactivated. In addition, it is difficult to adapt to some types of gas separation membranes, and there is a problem that it cannot be applied to various gas separation membranes.

Therefore, an object of the present invention is to provide a porous carbon fiber that can suppress adhesion between separation membranes and can be applied to supports of various gas separation membranes.

The present invention that solves the above-mentioned problems is as follows.

Porous carbon fiber characterized in that the fiber diameter fluctuates in the longitudinal direction and the CV% calculated from the fluctuation of the fiber diameter is 1% or more and 10% or less.

According to the porous carbon fiber of the present invention, it is possible to suppress the adhesion between the separation membranes and to provide the porous carbon fiber suitable for the support of various gas separation membranes.

The porous carbon fiber of the present invention is characterized in that the fiber diameter varies in the longitudinal direction, and the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less. Hereinafter, such porous carbon fibers will be described.

[Fiber diameter variation in the longitudinal direction]
In the porous carbon fiber of the present invention, the fiber diameter varies in the longitudinal direction, and it is important that the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less. Here, the fiber diameter refers to the diameter of a circle when the cross-sectional area of a cross section orthogonal to the fiber axis of an arbitrary porous carbon fiber is obtained and converted into a circle. At this time, if a void is included in the cross section such as a hollow cross section, the cross-sectional area calculated based on the outer circumference of the cross section including the void is used.

CV% refers to the value expressed as a percentage by dividing the standard deviation of the fiber diameter by the average fiber diameter. For the standard deviation of the fiber diameter and the value of the average fiber diameter, determine the cross section of any 300 or more porous carbon fibers among the porous carbon fibers, obtain the fiber diameter in each cross section, and calculate the total value of the fiber diameters. The average fiber diameter is divided by the measurement points, and the non-negative square root of the dispersion calculated from the fiber diameter at each measurement point and the obtained average fiber diameter is defined as the standard deviation of the fiber diameter.

The porous carbon fiber of the present invention may be used as it is, but it is preferable to form a separation membrane in a state where a separation functional layer having a function of separating gas is formed at least partially.

In the composite membrane for gas separation of the present invention, it is preferable to appropriately select a conventionally known substance as a substance constituting the separation functional layer having a function of separating gas according to the properties of the gas to be separated. At this time, the smaller the absolute value of the difference between the solubility parameter of the component that permeates the separation membrane and the solubility parameter of the substance that constitutes the separation functional layer, among the substances to be separated, the smoother the component that permeates the separation membrane permeates. Therefore, it is preferable because a device capable of efficient separation can be obtained. The absolute value of the difference in the solubility parameter is preferably 2.0 or less, more preferably 1.5 or less.

The larger the CV% of the porous carbon fiber of the present invention, the larger the change in the fiber diameter of the porous carbon fiber. Therefore, when the separation membranes are bundled, the separation membranes are spaced from each other, so that the gas flow path becomes larger. As well as being secured, the pressure drop due to pressure loss is alleviated, which enables efficient membrane separation. Further, at the time of modularization, since the potting agent is suppressed from being sucked up by the capillary phenomenon, the separation membrane surface is prevented from being blocked by the potting agent, and the decrease in the effective membrane area can be suppressed.

On the other hand, the smaller the CV% of the porous carbon fiber, the smaller and more uniform the change in the cross-sectional area of the fiber, so that the breakage due to stress concentration can be suppressed, so that the mechanical strength of the porous carbon fiber is high and the handleability is improved. There is. Further, when the gas separation functional layer is formed on the porous carbon fiber to form a gas separation composite film, the thickness of the gas separation functional layer can be uniformly laminated, so that defects in the gas separation functional layer can be suppressed.

From the above viewpoint, when the CV% calculated from the variation in the diameter of the porous carbon fibers is less than 1%, when the porous carbon fibers are bundled, the distance between the porous carbon fibers is small and the gas flow. The effect of reducing the pressure loss of the road and suppressing the suction of the potting agent cannot be sufficiently obtained. When the CV% exceeds 10%, breakage is likely to occur when handling the porous carbon fibers, and when a gas separation functional layer is formed on the porous carbon fibers to form a gas separation composite film, the gas separation functional layer is formed. Is likely to be defective. Therefore, the CV% calculated from the variation in the diameter of the porous carbon fiber is 1% or more and 10% or less, and preferably 1.5% or more and 7% or less.

Further, it is preferable that the porous carbon fiber of the present invention fluctuates by 3% or more and 10% or less in a section where the fiber diameter is 0.1 m in the longitudinal direction. Here, the fluctuation of the fiber diameter is obtained by dividing the porous carbon fiber having a length of 0.1 m into 10 at equal intervals, measuring the fiber diameter of the divided 11 points, and using the minimum diameter of the 11 points of measurement data as a reference. The rate of change to the maximum diameter was calculated as (maximum diameter-minimum diameter) / minimum diameter x 100 as a percentage, and the same operation was performed on 30 porous carbon fibers, and the average value of the obtained 30 points of change rate was obtained. More demanded.

As the fluctuation of the fiber diameter is larger, the distance between the separated membranes bundled per unit length is appropriately increased, so that the gas flow path is secured and the pressure drop due to pressure loss is alleviated, so that the pores are porous. When quality carbon fiber is applied to a gas separation membrane, it enables efficient membrane separation. Further, at the time of modularization, since the potting agent is suppressed from being sucked up by the capillary phenomenon, the separation membrane surface is prevented from being blocked by the potting agent, and the decrease in the effective membrane area can be suppressed. On the other hand, the smaller the variation in the fiber diameter, the less the extreme change in the fiber shape, so that the mechanical strength is high and the handleability is improved.

From the above viewpoint, it is more preferable that the fluctuation of the fiber diameter in the section of 0.1 m in the longitudinal direction is 3% or more and 7% or less.

Further, in the porous carbon fiber of the present invention, it is preferable that the fiber diameter periodically fluctuates in the longitudinal direction. For the periodicity of fluctuations in fiber diameter, a correlogram was created from the fiber diameters of porous carbon fibers measured in the longitudinal direction at 10 mm intervals, and the autocorrelation coefficient 0.2 in the range where the lugs displaced in the longitudinal direction were 50 mm or more. If there is more than that, it can be judged that there is periodicity. The period can also be determined from the value of the lag indicating that the autocorrelation coefficient is 0.2 or more. When the fiber diameter changes periodically as compared to the disordered fiber diameter change, the spacing between the bundled fibers is efficiently separated by the periodic fiber diameter, and the gas flow path is secured and the gas flow path is secured. Since the pressure drop due to pressure loss is alleviated, it is preferable to apply the porous carbon fiber to the gas separation membrane because it enables efficient membrane separation.

The smaller the period of periodic fluctuation of the fiber diameter in the longitudinal direction, the larger the distance between the bundled fibers because the average distance is separated at the thick part of the fiber diameter. On the other hand, the larger the cycle, the smaller the change in fiber diameter, and probably because it is possible to prevent the decrease in mechanical strength due to stress concentration, the mechanical strength is high and the handleability is improved.

When the porous carbon fiber of the present invention is applied to a separation membrane, the period in the longitudinal direction of the fiber diameter can be increased from the viewpoint of increasing the distance between the bundled separation membranes and providing a separation membrane having excellent mechanical strength. The period of fluctuation is preferably 20 mm or more and 10000 mm or less, and more preferably 20 mm or more and 3000 mm or less.

[Porous carbon fiber]
The porous carbon fiber of the present invention refers to a carbon fiber containing many pores. The pores may be open to the outside or may exist as a space inside without opening. The porous carbon fiber of the present invention preferably has a co-continuous structure at least partially, and the co-continuous structure may be continuous to the outer surface of the porous carbon fiber so that the outer surface may be opened, or vice versa. In addition, the outer surface may be blocked by interrupting the co-continuous structure up to the outer surface of the porous carbon fiber. The co-continuous structure is a structure in which the branches and pores (voids) of the carbon skeleton are continuously entwined in three dimensions. Specifically, a sample sufficiently cooled in liquid nitrogen is set with twill. When the cross section cut by the above is observed on the surface with a scanning electron microscope, it can be confirmed by observing how the branches and voids of the carbon skeleton are continuously entangled with each other. Further, the fact that the porous carbon fiber has a co-continuous structure means that such a co-continuous structure is observed in an arbitrary cross section of the porous carbon fiber.

When the porous carbon fibers have a co-continuous structure, the carbon skeletons are three-dimensionally continuous, so that the carbon skeletons have the effect of supporting the entire structure and stress can be dispersed throughout the porous carbon fibers. It becomes possible to have great resistance to external forces such as compression and bending, that is, to have high compression strength and compression specific strength. Further, since the voids are also three-dimensionally communicated with each other, the voids can function as a gas flow path.

Examples of the co-continuous structure include a lattice shape and a monolith shape, and are not particularly limited. However, in terms of exhibiting the above effects, the monolith shape tends to improve the compressive strength in the fiber cross-sectional direction, and is therefore preferable. .. The monolithic shape refers to a form in which the carbon skeleton is three-dimensionally and uniformly continuous in a co-continuous structure, and is a structure in which individual particles are aggregated and linked, or conversely, a template particle in which individual particles are aggregated and linked. It is distinguished from irregular structures such as those formed by the voids created by removal and the surrounding skeleton, or continuous structures of biological cell walls.

The structural period of the co-continuous structure of the porous carbon fiber is preferably 0.002 μm or more and 20 μm or less. The fact that the porous carbon fibers have a structural period of the co-continuous structure indicates that the uniformity of the co-continuous structure is high, and it means that the thickness and the pore size of the branches of the carbon skeleton are uniform. As a result, the effect of improving the compressive strength of the gas separation membrane can be obtained. When the structural period of the co-continuous structure is 20 μm or less, the carbon skeleton and the pores become a fine structure and the compressive strength is improved. Therefore, the structural period is more preferably 10 μm or less, further preferably 5 μm or less. On the other hand, when the structural period of the co-continuous structure is 0.002 μm or more, the pressure loss when the gas is passed through the voids is reduced and the gas permeation rate is improved. In addition, when the pressure loss is reduced, it has the effect of being able to separate and purify with more energy saving. Therefore, the structural period is more preferably 0.02 μm or more, and further preferably 0.1 μm or more.

The structural period of the co-continuous structure is calculated by the following formula from the scattering angle 2θ at the position of the peak top of the scattering intensity obtained by incident X-ray on the porous carbon fiber and scattering it at a small angle.

L = λ / (2sinθ)
L: Structural period, λ: Wavelength of incident X-rays However, there are cases where X-ray scattering at small angles cannot be observed due to the large structural period. In that case, the structural period is obtained by X-ray computer tomography (X-ray CT). Specifically, after Fourier transforming a three-dimensional image taken by X-ray CT, the annular average of the two-dimensional spectrum is taken to obtain a one-dimensional spectrum. The characteristic wavelength corresponding to the position of the peak top in the one-dimensional spectrum is obtained, and the structural period is calculated as the reciprocal of the characteristic wavelength. At this time, those in which a plurality of peaks are observed are not suitable for calculating the structural period of the co-continuous structure of the present application. Generally, when multiple peaks are observed, it is a case where the structure has a very high crystallinity. For example, microphase separation and mesoporous silica as a template are exemplified, but the co-continuous structure of the present application is this. Is clearly different.

If the average diameter of the entire pores forming the co-continuous structure of the porous carbon fibers is too small, the pressure loss increases and the gas permeability decreases, so that the average diameter is preferably 30 nm or more, and more preferably 100 nm or more. Further, if the average diameter of the entire pores is too large, the effect of the carbon branches supporting the entire structure is reduced and the compressive strength is lowered. Therefore, 5,000 nm or less is preferable, and 2,500 nm or less is more preferable. preferable.

The average diameter of the entire pore is a measured value by measuring the pore diameter distribution by the mercury intrusion method. In the mercury injection method, pressure is applied to pores having a co-continuous structure to infiltrate mercury, and the pore volume and specific surface area are obtained from the pressure and the amount of injected mercury. Then, the pore diameter obtained from the relationship between the pore volume and the specific surface area is calculated when the pores are assumed to be a cylinder, and the pore diameter distribution curve of 5 nm to 500 μm can be obtained by the mercury intrusion method.

When the separation functional layer is formed on the porous carbon fiber of the present invention, it is preferable that the outer surface of the porous carbon fiber, that is, the interface with the separation functional layer of the porous carbon fiber is opened. When the pores are open at the interface with the separation functional layer, the pressure loss when the porous carbon fiber or vice versa permeates from the separation functional layer is reduced, so that the gas permeates in the gas separation membrane. The speed can be improved. In addition, since the outer surface of the porous carbon fiber is uneven, the adhesive effect with the separation function layer is improved, and peeling during use is suppressed to obtain a gas separation membrane with excellent durability. Be done.

In the porous carbon fiber, the gas permeation rate of the gas separation membrane increases as the pore diameter of the pores at the interface with the separation functional layer increases, so the average pore diameter is preferably 2 nm or more, more preferably 10 nm or more. , 50 nm or more is more preferable. On the other hand, if the pore diameter is too large, the inorganic material may penetrate deep inside the porous carbon fiber when forming the separation functional layer, and may not be uniformly laminated on the surface. Therefore, the average pore diameter Is preferably 500 nm or less, more preferably 400 nm or less, still more preferably 300 nm or less.

Here, the fact that the pores are open at the interface with the separation functional layer means that an arbitrary cross section of the porous carbon fiber is precisely prepared by an ion milling device or the like and is porous when observed with an electron microscope. A state in which a portion of the carbon fiber in which the pores and the interface are in direct contact is observed. The average pore diameter is measured along the interface from the contact point between one carbon and the interface to the contact point of the interface portion where the voids, which are the pores of the porous carbon fiber, and the interface are in direct contact with each other. The length is measured at any 10 points and calculated based on the average value.

The finer the diameter of the porous carbon fiber, the more flexible it is, the more resistant it is to breakage, and the more it can withstand high pressure, the more preferable it is. From these points, the average value of the fiber diameters of the porous carbon fibers is preferably in the range of 20 μm or more and 5,000 μm or less.

When the porous carbon fiber is a fiber having a hollow portion, that is, a hollow fiber, the lower the hollow ratio is, the higher the pressure resistance is preferable, and the higher the hollow ratio is, the more the gas pressure loss can be reduced. preferable. From these points, the hollow ratio is preferably in the range of 1% or more and 90% or less, and more preferably in the range of 5% or more and 60% or less.

<Manufacturing method of porous carbon fiber, composite membrane for gas separation using it, and module for gas separation>
The porous carbon fiber of the present invention is, as an example,
A step (step 1) in which a carbonizable resin and a vanishing resin are compatible with each other to form a resin mixture;
The step of phase-separating the resin mixture in a compatible state and forming it into a fibrous form (step 2);
With the step of removing the vanishing resin from the phase-separated resin mixture (step 3);
A step of carbonizing by heating to obtain porous carbon fibers (step 4);
It can be manufactured by the manufacturing method having.

As an example, the composite membrane for gas separation of the present invention further forms a separation functional layer on porous carbon fibers to obtain a composite membrane for gas separation (step 5).
It can be manufactured by the manufacturing method having.

The gas separation module of the present invention is further described as an example.
Step of accommodating the composite membrane for gas separation and obtaining a module for gas separation (step 6);
It can be manufactured by the manufacturing method having.

[Step 1] Composite mixing of carbonizable resin and vanishing resin Step 1 is a step of compatibilizing the carbonizable resin and vanishing resin to form a resin mixture. Here, the carbonizable resin is a resin that is carbonized by heating and remains as a branch portion (carbon skeleton), and both a thermoplastic resin and a thermosetting resin can be used.

In the case of a thermoplastic resin, it is preferable to select a resin that can be infusible by a simple process such as heating or high energy ray irradiation. Further, in the case of a thermosetting resin, an infusibilizing treatment is often unnecessary, and this is also mentioned as a suitable material.

Examples of the thermoplastic resin include polyphenylene ether, polyvinyl alcohol, polyacrylonitrile, phenol resin, total aromatic polyester, polyimide resin, cellulose acetate and polyetherimide, and examples of the thermosetting resin include unsaturated polyester. Resins, alkyd resins, melamine resins, urea resins, polyimide resins, diallyl phthalate resins, lignin resins, urethane resins, polyfurfuryl alcohol resins and the like can be listed. These may be used alone or in a mixed state, but it is also preferable to mix them with a thermoplastic resin or a thermosetting resin from the viewpoint of ease of molding.

Among them, from the viewpoint of carbonization yield, spinnability, and economy, it is preferable to use a thermoplastic resin, and polyphenylene ether, polyvinyl alcohol, polyacrylonitrile, and total aromatic polyester are more preferably used.

The molecular weight of the carbonizable resin is preferably 10,000 or more in terms of weight average molecular weight. When the weight average molecular weight is 10,000 or more, yarn breakage is reduced in the process of forming into a spinning yarn. On the other hand, the upper limit of the weight average molecular weight is not particularly limited, but is preferably 1,000,000 or less from the viewpoint of spinnability / moldability and easy extrusion of the resin.

The vanishing resin is a resin that can be removed at any stage after the formation of the phase-separated structure in step 2 described later.

The method for removing the vanishing resin is not particularly limited, and is chemically removed by depolymerizing with a chemical, a method of adding a solvent for dissolving the vanished resin to dissolve and remove it, and a method of thermally decomposing by heating. A method of reducing the molecular weight of the vanished resin and removing the resin is preferably used. These methods can be carried out individually or in combination, and when they are carried out in combination, they may be carried out simultaneously or separately.

As a method for chemically removing, a method of hydrolyzing with an acid or an alkali is preferable from the viewpoint of economy and handleability. Examples of the resin susceptible to hydrolysis by acid or alkali include polyester, polycarbonate, polyamide and the like.

As a method of adding and removing a solvent for dissolving the disappearing resin, a method of continuously supplying a solvent to the mixed carbonizable resin and the disappearing resin to dissolve and remove the disappearing resin, or a batch method is used. A preferred example is a method of mixing to dissolve and remove the vanishing resin.

Specific examples of the vanishing resin suitable for the method of adding and removing a solvent include polyolefins such as polyethylene, polypropylene and polystyrene, acrylic resin, methacrylic resin, polyvinylpyrrolidone, aliphatic polyester and polycarbonate. Among them, an amorphous resin is more preferable because of its solubility in a solvent, and examples thereof include polystyrene, methacrylic resin, and polycarbonate.

As a method of reducing the molecular weight of the vanishing resin by thermal decomposition and removing it, a method of thermally decomposing the mixed carbonizable resin and the vanishing resin by heating them in a batch manner, or a method of thermally decomposing the mixed carbonizable resin and the vanishing resin as a heating source. An example is a method of thermally decomposing by heating while continuously supplying the inside.

The vanishing resin is preferably a resin that disappears by thermal decomposition when the carbonizable resin is carbonized by heating in step 4 described later, does not cause a large chemical change during the infusibilization treatment described later, and is after heating. It is preferably a thermoplastic resin having a carbonization yield of less than 10%.

Specific examples of such vanishing resins include polyolefins such as polyethylene, polypropylene, and polystyrene, acrylic resins, methacrylic resins, polyacetal, polyvinylpyrrolidone, aliphatic polyesters, aromatic polyesters, aliphatic polyamides, and polycarbonates. These can be used alone or in a mixed state.

In step 1, the carbonizable resin and the vanishing resin are compatible with each other to form a resin mixture (polymer alloy). The term "compatible" as used herein means to create a state in which the phase separation structure of the carbonizable resin and the vanishing resin is not observed with an optical microscope by appropriately selecting the temperature and / or solvent conditions.

The carbonizable resin and the vanishing resin may be compatible with each other by mixing only the resins, or may be further compatible with each other by adding a solvent.

As a system in which a plurality of resins are compatible with each other, a system showing a phase diagram of the upper limit critical eutectic temperature (UCST) type, which is in a phase-separated state at a low temperature but becomes one phase at a high temperature, or conversely, a phase-separated state at a high temperature. However, there is a system showing a lower limit critical eutectic temperature (LCST) type phase diagram, which is one phase at low temperature.

Further, particularly when at least one of the carbonizable resin and the vanishing resin is a system dissolved in a solvent, a system in which phase separation described later is induced by permeation of a non-solvent can be mentioned as a preferable example.

The solvent to be added is not particularly limited, but the absolute value of the difference from the average value of the solubility parameter (SP value) of the carbonizable resin and the vanishing resin, which is an index of solubility, is preferably 5.0 or less.

It is known that the smaller the absolute value of the difference from the average SP value, the higher the solubility, so it is preferable that there is no difference. Further, the larger the absolute value of the difference from the average SP value is, the lower the solubility becomes, and it becomes difficult to take a compatible state between the carbonizable resin and the vanishing resin. From this, the absolute value of the difference from the average SP value is preferably 3.0 or less, more preferably 2.0 or less.

As an example of a specific combination of a carbonizable resin and a vanishing resin in a compatible system, if the system does not contain a solvent, polyphenylene ether / polystyrene, polyphenylene ether / styrene-acrylonitrile copolymer, total aromatic polyester / polyethylene Examples include terephthalate, total aromatic polyester / polyethylene naphthalate, and total aromatic polyester / polycarbonate, and if compatibility is difficult from the viewpoint of degree of polymerization and stereoregularity, copolymerization or chemical modification is appropriate. It is also preferable to perform quality and to make the combination of both compatible.

Specific examples of combinations of systems containing a solvent include polyacrylonitrile / polyvinyl alcohol, polyacrylonitrile / polyvinylphenol, polyacrylonitrile / polyvinylpyrrolidone, polyacrylonitrile / polylactic acid, polyvinyl alcohol / vinyl acetate-vinyl alcohol copolymer, and polyvinyl. Examples include alcohol / polyethylene glycol, polyvinyl alcohol / polypropylene glycol, polyvinyl alcohol / starch, etc., and if compatibility is difficult from the viewpoint of degree of polymerization and stereoregularity, copolymerization or chemical modification is appropriately performed. It is also preferable to carry out the copolymerization of the combination of the two.

The method of mixing the carbonizable resin and the vanishing resin is not limited, and various known mixing methods can be adopted as long as they can be mixed uniformly. Specific examples include a rotary mixer having a stirring blade and a kneading extruder using a screw.

It is also preferable that the temperature (mixing temperature) when the carbonizable resin and the vanishing resin are mixed is set to a temperature equal to or higher than the temperature at which both the carbonizable resin and the vanishing resin soften. Here, as the temperature for softening, the melting point may be appropriately selected if the carbonizable resin or the vanishing resin is a crystalline polymer, and the glass transition temperature may be appropriately selected if the resin is an amorphous resin.

By setting the mixing temperature to a temperature higher than the temperature at which both the carbonizable resin and the vanishing resin soften, the viscosity of both can be lowered, so that more efficient stirring and mixing become possible. The upper limit of the mixing temperature is also not particularly limited, but is preferably 400 ° C. or lower from the viewpoint of preventing deterioration of the resin due to thermal decomposition and obtaining porous carbon fibers having excellent quality.

Further, in step 1, it is preferable to mix 90 to 10% by weight of the vanishing resin with 10 to 90% by weight of the carbonizable resin. When the carbonizable resin is 10% by weight or more, it is possible to maintain the porous carbon fibers after carbonization and the yield is improved, which is preferable. Further, when the carbonizable resin is 90% by weight or less, the vanishing resin can efficiently form voids, which is preferable.

The mixing ratio of the carbonizable resin and the vanishing resin can be arbitrarily selected in consideration of the compatibility of each resin. Specifically, in general, the compatibility between resins deteriorates as the composition ratio approaches 1: 1. Therefore, when a system having less compatibility is selected as a raw material, the amount of carbonizable resin is increased. Alternatively, it is also preferable to improve the compatibility by reducing the composition to bring it closer to the so-called biased composition.

It is also preferable to add a solvent when mixing the carbonizable resin and the vanishing resin. By adding a solvent, the viscosity of the carbonizable resin and the vanishing resin is lowered, the molding is facilitated, and the carbonizable resin and the vanishing resin are easily compatible with each other.

The solvent referred to here is not particularly limited as long as it is a liquid at room temperature capable of dissolving and swelling at least one of the carbonizable resin and the vanishing resin, and the carbonizable resin and the vanishing resin may be used. If it also dissolves, it is more preferable because it is possible to improve the compatibility between the two.

The amount of the solvent added is preferably 20% by weight or more with respect to the total weight of the carbonizable resin and the vanishing resin from the viewpoint of improving the compatibility between the carbonizable resin and the vanishing resin, lowering the viscosity and improving the fluidity. On the other hand, from the viewpoint of cost associated with recovery and reuse of the solvent, 90% by weight or less is preferable with respect to the total weight of the carbonizable resin and the vanishing resin.

[Step 2] Phase Separation / Molding Step 2 is a step of phase-separating the resin mixture in the phased state in step 1 and molding a precursor fiber of a porous carbon fiber whose fiber diameter varies in the longitudinal direction. be.

The method of forming the resin mixture in a compatible state into a fibrous form is not particularly limited, and a spinning method suitable for the phase separation method described later can be appropriately selected. If the resin mixture is a combination of thermoplastic resins, melt spinning can be performed after heating to a temperature equal to or higher than the softening temperature of the resin. When the resin mixture contains a solvent, dry spinning, dry wet spinning, wet spinning and the like can be appropriately selected as the solution spinning.

Melt spinning is a method of extruding a resin mixture heated and melted (flowing state) using a kneading extruder or the like from a mouthpiece and winding it while cooling, and the process speed is faster than that of solution spinning. Excellent productivity. Further, since the solvent does not volatilize, the cost for safety measures during the process can be suppressed, and the production can be performed at low cost, which is preferable.

In addition, solution spinning is a method in which a spinning dope composed of a resin mixture and a solvent prepared in advance is weighed and extruded from a mouthpiece to form fibers, and the phase separation state can be precisely controlled.

In particular, for dry-wet spinning and wet spinning using a coagulation bath, the phase separation state of the precursor fiber can be precisely controlled by appropriately combining heat-induced phase separation and non-solvent-induced phase separation, which will be described later. be.

The method of forming into a fibrous form and separating the carbonizable resin and the vanishing resin into a phase separation is not particularly limited. A non-solvent-induced phase separation method may be mentioned.

These phase separation methods can be applied alone or in combination. Specific methods when applied in combination include, for example, a method of causing non-solvent-induced phase separation through a coagulation bath and then heating to cause heat-induced phase separation, or a method of controlling the temperature of the coagulation bath to induce non-solvent induction. Examples thereof include a method of simultaneously causing phase separation and heat-induced phase separation, and a method of cooling the resin discharged from the mouthpiece to cause heat-induced phase separation and then contacting the resin with a non-solvent.

At this time, the phase separation condition can be arbitrarily selected from the size of the structural cycle of the obtained porous carbon fiber, but it is preferable to appropriately select the temperature and the composition of the resin mixture in a phased state. Although it is related to the step of immobilizing the phase separation structure described later, the closer the temperature and the composition of the resin mixture in the phased state are to the conditions for achieving the phase separation state, the more difficult the phase separation tends to proceed. Yes, the farther it is, the easier it is for phase separation to proceed. By appropriately selecting these conditions, a precursor for obtaining a desired porous carbon fiber can be produced.

Further, after passing through a coagulation bath, it is washed with water and dried to form and immobilize a phase-separated structure, and fibers that are precursors of porous carbon fibers can be obtained. Here, the coagulating liquid is not particularly limited, and examples thereof include water, ethanol, an aqueous salt solution, and a mixed solvent of these and the solvent used in step 1.

The method for obtaining the porous carbon fiber of the present invention in which the fiber diameter varies in the longitudinal direction and the CV% calculated from the variation in the fiber diameter is 1% or more and 10% or less is not particularly limited, but the fiber diameter is longitudinal. A method for obtaining directionally variable precursor fibers is preferably used. The method for forming the precursor fiber whose fiber diameter varies in the longitudinal direction is not particularly limited, and for example, a method for changing the discharge amount from the mouthpiece, a method for changing the roll speed of the process, and a method for stretching with hot water. , How to utilize the draw resonance phenomenon. These molding methods may be applied alone or in combination of two or more. In particular, for these molding methods, it is preferable to prepare conditions for periodically causing fluctuations in the fiber diameter.

The method for obtaining the porous carbon fiber of the present invention in which the fiber diameter periodically fluctuates in the longitudinal direction and the porous carbon fiber of the present invention in which the cycle of the periodic fluctuation in the fiber diameter in the longitudinal direction is 20 mm or more and 10,000 mm or less is particularly limited. However, a method for obtaining a precursor fiber in which the fiber diameter periodically fluctuates in the longitudinal direction and a precursor fiber in which the cycle of the periodic fluctuation in the fiber diameter in the longitudinal direction is 20 mm or more and 10,000 mm or less is preferably used. The conditions for periodically changing the fiber diameter of the precursor fiber are not particularly limited, but by periodically changing the discharge amount from the base or the roll speed of the process by a method using an inverter motor or an eccentric roll. , Precursor fibers in which the fiber diameter fluctuates periodically can be obtained. Similarly, in the method utilizing the draw resonance phenomenon, the period can be controlled by adjusting the draft ratio, and a precursor fiber whose fiber diameter fluctuates periodically to some extent can be obtained.

These methods can be applied alone or in combination. The process of forming the precursor fiber whose fiber diameter varies in the longitudinal direction is described as step 2, but it is not always necessary to perform the process in step 2, and the infusibilizing treatment (step 3) and carbonization treatment (step 4) described later are not necessary. It may be done at the same time. It was

[Step 3] Removal of vanishing resin The precursor fiber of the porous carbon fiber obtained in Step 2 is before being subjected to the carbonization step (Step 4), at the same time as the carbonization step (Step 4), or in the carbonization step (Step 4). At least at one time point after 4), it is subjected to the removal treatment of the vanishing resin (step 3). That is, although the process of removing the vanishing resin is described as "step 3" for convenience of explanation, the step 3 does not necessarily have to be performed before the post-step 4 of the step 2, and is actually performed at the same time as the step 4. However, it may be performed after step 4. Further, it may be performed at the same time as the infusibilization treatment described later.

The method of removing the lost resin is not particularly limited. Specifically, a method of chemically decomposing and removing the vanishing resin using an acid, an alkali, an enzyme, and oxygen to reduce the molecular weight and removing the resin, a method of dissolving and removing the vanishing resin with a solvent that dissolves the vanishing resin, electron beam, gamma ray, and ultraviolet rays. , A method of decomposing and removing the vanishing resin by using radiation such as infrared rays or heat.

In particular, when the vanishing resin can be removed by thermal decomposition, heat treatment can be performed at a temperature at which 80% by weight or more of the vanished resin disappears before the carbonization treatment (step 4), or the carbonization treatment (step 4) can be performed. The vanishing resin can also be thermally decomposed and gasified to be removed in step 4) or in the infusible treatment described later. It is preferable to thermally decompose and gasify the vanishing resin at the same time as the heat treatment in the carbonization treatment (step 4) or the infusibilization treatment described later because the productivity is increased.

[Infusibilization treatment]
The precursor fiber of the porous carbon fiber is preferably subjected to an infusibilization treatment at any stage after the phase separation (step 2) and before being subjected to the carbonization treatment (step 4).

The method of infusibilization treatment is not particularly limited, and a known method can be used. Specific methods include a method of causing oxidative cross-linking by heating in the presence of oxygen, a method of irradiating high-energy rays such as electron beams and gamma rays to form a cross-linked structure, and impregnation with a substance having a reactive group. Examples thereof include a method of mixing to form a crosslinked structure, and among them, a method of causing oxidative crosslinking by heating in the presence of oxygen is preferable because the process is simple and the manufacturing cost can be kept low. These methods may be used alone or in combination, or each may be used at the same time.

The heating temperature in the method of causing oxidative cross-linking by heating in the presence of oxygen is preferably 150 ° C. or higher from the viewpoint of efficiently advancing the cross-linking reaction, and the yield deteriorates due to weight loss due to thermal decomposition, combustion, etc. of the carbonizable resin. From the viewpoint of preventing the above, 350 ° C. or lower is preferable.

The oxygen concentration during the infusibilization treatment is not particularly limited, but it is preferable to supply a gas having an oxygen concentration of 18% by volume or more because the production cost can be kept low. The method of supplying the gas is not particularly limited, and examples thereof include a method of supplying air as it is into the heating device and a method of supplying pure oxygen into the heating device using a cylinder or the like.

As a method of irradiating a high energy ray such as an electron beam or a gamma ray to form a crosslinked structure, a commercially available electron beam generator or gamma ray generator is used to irradiate the carbonizable resin with an electron beam or a gamma ray. Then, there is a method of inducing cross-linking.

From the viewpoint of efficient introduction of the crosslinked structure by irradiation, the lower limit of the irradiation intensity is preferably 1 kGy or more, and it is possible to prevent the strength of the precursor fiber of the porous carbon fiber from decreasing due to the decrease in molecular weight due to the cleavage of the main chain. From the viewpoint, 1,000 kGy or less is preferable.

As a method of impregnating and mixing a substance having a reactive group to form a crosslinked structure, a low molecular weight compound having a reactive group is impregnated into a resin mixture and heated or irradiated with high energy rays to proceed with the crosslinking reaction. Examples thereof include a method in which a low molecular weight compound having a reactive group is mixed in advance and then heated or irradiated with high energy rays to proceed with a crosslinking reaction.

[Step 4] Carbonization In Step 4, the precursor fiber of the porous carbon fiber obtained in Step 2 or, if necessary, the precursor fiber subjected to the removal and / or infusibilization treatment of the vanishing resin is carbonized by heating. This is a step of treating to obtain porous carbon fiber.

In order to carbonize the precursor fiber of the porous carbon fiber, it is preferable that the carbonization treatment in this step is performed by heating by heat conduction or microwave heating in an inert gas atmosphere.

Here, the inert gas refers to a gas that is chemically inert when heated, and specific examples thereof include helium, neon, nitrogen, argon, krypton, xenon, and carbon dioxide. Above all, it is preferable to use nitrogen and argon from the economical point of view.

The flow rate of the inert gas may be an amount that can sufficiently reduce the oxygen concentration in the heating device, and it is preferable to appropriately select an optimum value according to the size of the heating device, the supply amount of the raw material, the heating temperature, and the like. ..

The upper limit of the flow rate is not particularly limited, but it is preferable to set it appropriately according to the temperature distribution and the design of the heating device from the viewpoint of economy and reducing the temperature change in the heating device.

Further, it is more preferable that the gas generated during carbonization can be sufficiently discharged to the outside of the system because it is possible to obtain porous carbon fibers having excellent quality. From this, it is preferable to determine the flow rate of the inert gas so that the concentration of the generated gas in the system is 3,000 ppm or less.

When heating by heat conduction, the heating temperature is preferably 300 ° C. or higher, more preferably 400 ° C. or higher. Further, although the upper limit of the heating temperature is not limited, if the temperature is 1,500 ° C. or lower, no special processing is required for the equipment, which is preferable from an economical point of view. When the above-mentioned removal of the disappearing resin (step 3) is performed at the same time, it is preferable to heat the resin to a temperature higher than the temperature at which the disappearing resin is thermally decomposed.

In this step, the precursor fiber of the porous carbon fiber can be cut and heated in a batch manner with a heating device, but it is more preferable to continuously heat the precursor fiber without cutting it. The method of continuous heating is not particularly limited, and examples thereof include heating by heat conduction and microwave heating. Since these heating methods are methods in which the precursor fibers of the porous carbon fibers are continuously supplied into the heating device by using a roller, a conveyor, or the like and taken out, it is possible to increase the productivity. preferable.

The rate of temperature increase and decrease in the case of batch processing in the heating device is not limited, and productivity can be increased by shortening the time required for temperature increase and decrease, so that the temperature is 1 ° C./min or more. Speed is preferred. Further, the upper limit of the temperature rising rate and the temperature lowering rate is not particularly limited, and can be appropriately set within a range in which defects such as cracks do not occur.

[Step 5] Gas Separation Composite Membrane The gas separation composite membrane of the present invention is a membrane having the porous carbon fiber and the separation functional layer of the present invention. Therefore, the step 5 for obtaining the gas separation composite film is a step of forming a separation functional layer on the porous carbon fiber obtained in the step 4 to form a gas separation composite film. If the porous carbon fiber is not used as the gas separation composite membrane, step 5 can be omitted.

The type of the separation functional layer is not particularly limited, and examples thereof include a polymer membrane, a zeolite membrane, a silica membrane, and a carbon membrane.

A known method can be adopted as the method for forming the separation functional layer. As a general forming method, a resin coating is used for a high-separation film, hydrothermal synthesis is used for a zeolite film, a sol-gel method or an opposed diffusion method is used for a silica film, and an infusibilizing and carbonization heat treatment method is used for a carbon film after resin coating. Be done. Examples of the coating method for porous carbon fibers include a dip coating method, a nozzle coating method, a spray method, a vapor deposition method, and a cast coating method. The dip coating method or the nozzle coating method is preferable because of the ease of manufacturing method.

[Step 6] Gas Separation Module The gas separation module of the present invention is a module containing the gas separation composite membrane of the present invention. Therefore, the step 6 for obtaining the gas separation module is a step of bundling the gas separation composite membrane obtained in the step 5 and accommodating it in the vessel. If the gas separation composite membrane is not used as the gas separation module, step 6 can be omitted.

The gas separation composite membrane obtained in step 5 is bundled, housed in an element casing (hereinafter referred to as casing), fixed with a potting agent, and both ends of the casing are sealed. Examples of the potting method include a centrifugal potting method in which centrifugal force is used to permeate a gas separation composite membrane, and a static potting method in which a flowing potting material is sent by a metering pump or a head and permeated into the gas separation composite membrane. The law etc. can be mentioned.

It is preferable to cut the potted gas separation composite membrane at the potting site to open the end of the gas separation composite membrane. One or more casings provided with the obtained composite membrane for gas separation can be housed in a vessel to produce a gas separation module.

Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto. The evaluation in each Example and Comparative Example was performed by the following method.

(CV% of fiber diameter)
After determining the cross sections of 300 porous carbon fibers among the porous carbon fibers, the fiber diameter was determined for each cross section using an electron microscope (S 5500 type; Hitachi High-Technologies Co., Ltd.), and the obtained data CV% was calculated from the average fiber diameter and standard deviation.

(Variation of fiber diameter)
Porous carbon fiber with a length of 0.1 m is divided into 10 at equal intervals, the fiber diameters of the divided 11 points are measured, and the rate of change to the maximum diameter is determined based on the minimum diameter of the 11 points of measurement data. It was calculated as a percentage as (maximum diameter-minimum diameter) / minimum diameter x 100. The same operation was performed on 30 porous carbon fibers, and the fluctuation of the fiber diameter was obtained from the average value of the obtained change rates of 30 points.

(Periodic fluctuation of fiber diameter)
The periodicity was evaluated by preparing a correlogram from the data obtained by determining the fiber diameter of the porous carbon fiber at fiber length intervals of 10 mm, and evaluating the data by the autocorrelation coefficient in a lug shifted by 50 mm or more. When the autocorrelation coefficient has a value of 0.2 or more, it is determined that there is a periodic variation in the longitudinal direction, and in other cases, it is determined that there is no periodic variation in the longitudinal direction. In addition, the period was determined from the value of the lag showing an autocorrelation coefficient of 0.2 or more.

(Structural period of co-continuous structure)
Five porous carbon fibers were sandwiched between sample plates, and the positions of the light source, sample and two-dimensional detector were adjusted so that information with a scattering angle of less than 10 degrees could be obtained from the X-ray source obtained from the CuKα ray light source. From the image data (luminance information) obtained from the two-dimensional detector, the central part affected by the beam stopper is excluded, the radius is set from the center of the beam, and the brightness value of 360 ° is set for every 1 ° angle. The scattering intensity distribution curve was obtained by adding up. From the scattering angle θ at the position having a peak in the obtained curve, the structural period of the continuous structure portion was calculated by the following formula, and the average value of five porous carbon fibers was obtained as the structural period of the co-continuous structure.

When the structural period was 1 μm or more and the peak of X-ray scattering was not observed, a continuous rotation image was taken in a range of 180 ° or more in 0.3 ° steps with an X-ray microscope to obtain a CT image. A Fourier transform was performed on the obtained CT image to obtain a graph of the scattering angle θ and the scattering intensity, and the structural period was obtained by the following formula by the same method as described above.

L = λ / (2sinθ)
L: Structural period, λ: Wavelength of incident X-rays (evaluation of film performance)
Twenty 30 cm long gas separation composite membranes are bundled and housed in a stainless steel casing with an outer diameter of φ6 mm and a wall thickness of 1 mm, and the ends of the bundled gas separation composite membranes are placed on the inner surface of the casing with an epoxy resin adhesive. It was fixed and both ends of the casing were sealed. Then, the potted gas separation composite membrane was cut at the potting site to open the end of the gas separation composite membrane. A module was prepared by housing a casing with 20 gas separation composite membranes in a stainless steel vessel, and the gas permeation rate was measured. Carbon dioxide and methane are used as the measurement gas, and the pressure change on the permeation side per unit time of carbon dioxide and methane is measured by an external pressure method at a measurement temperature of 25 ° C. in accordance with the pressure sensor method of JIS K7126-1 (2006). bottom. Here, the pressure difference between the supply side and the permeation side was set to 0.11 MPa (82.5 cmHg).

Subsequently, the permeation rate Q of the permeated gas was calculated by the following formula, and the separation coefficient α was calculated as the ratio of the permeation rate of the gas of each component. In addition, STP means a standard condition. The membrane area was calculated from the outer diameter and length of the gas separation composite membrane in the region contributing to gas permeation.

Q = [Gas permeation flow rate (× 10 ^-6 cm ³ · STP)] / [Membrane area (cm ² ) × time (s) × pressure difference (cmHg)
When the separation coefficient α was a value larger than 1.0, it was determined to have separability "yes", and in other cases, it was determined to have separability "none".

(Sucking up potting material)
A gas separation composite membrane having a porous carbon fiber and a separation functional layer is suspended in a bundle of 100 each, and a potting material (potting material) so as to immerse up to 1 cm from the lower end of the gas separation composite membrane bundle. Epoxy resin) was injected. After the potting material is cured by allowing it to stand in a constant temperature bath at a temperature of 50 ° C. for 12 hours, the bundle is unwound from the upper end side, and the part that cannot be unraveled (the part where all the separation membranes are adhered by sucking up the potting material) is potted. It was the destination of the material. The distance between the hardened surface of the potting material and the reaching point was measured and used as the suction height of the potting material.

[Example 1]
Polyacrylonitrile (MW 150,000) and polyvinylpyrrolidone (MW 40,000) and dimethyl sulfoxide (DMSO) as a solvent were put into a separable flask, and the ratio of polyacrylonitrile and polyvinylpyrrolidone was 1: 1 and the polymer concentration was 20% by weight. A uniform and transparent solution was prepared with stirring and refluxing.

The obtained polymer solution was discharged from the outer tube of the core-sheath type double mouthpiece, and the DMSO aqueous solution was simultaneously discharged from the inner tube, and then led to a mixed bath of water and DMSO. After passing through a mixed bath roll whose speed changes in a cycle of 10 s, winding was performed to obtain a hollow thread-like PAN-based precursor fiber. The obtained PAN-based precursor fiber was washed with water and then dried.

After that, infusibilization treatment was performed in an air atmosphere to produce infusible fibers.

Subsequently, the PAN-based precursor fiber was carbonized at an ultimate temperature of 700 ° C. to prepare a porous carbon fiber.

The obtained porous carbon fiber is immersed in a polyacrylonitrile / DMSO solution (polymer 10% by weight), pulled up, immersed in water to remove the solvent, and dried at 100 ° C. for 24 hours on the porous carbon fiber. A laminate having a resin layer of polyacrylonitrile was prepared. After that, the laminate was infused in an air atmosphere. Subsequently, the infusible yarn was carbonized to prepare a hollow yarn-shaped composite membrane for gas separation. Table 1 shows the evaluation results of the porous carbon fiber and the composite membrane for gas separation.

[Example 2]
Porous carbon fiber and gas separation are performed by the same method as in Example 1 except that draw resonance is expressed and hollow filamentous PAN-based precursor fiber is obtained by adjusting the base shape, discharge speed, and mixed bath roll rotation speed. A composite film was prepared. Table 1 shows the evaluation results of the obtained porous carbon fiber and the composite membrane for gas separation.

[Comparative Example 1]
Porous carbon fibers and a composite membrane for gas separation were produced by the same method as in Example 1 except that hollow filament-shaped PAN-based precursor fibers were obtained under the condition that the mixed bath roll was rotated at a constant speed. Table 1 shows the evaluation results of the obtained porous carbon fiber and the composite membrane for gas separation.

[Comparative Example 2]
Porous carbon fibers and a composite film for gas separation were prepared by the same method as in Example 1 except that hollow filament-like composite fibers were obtained under the condition that the fluctuation width of the speed of the mixed bath roll whose speed changed in a 10s cycle was increased. .. The evaluation results are shown in Table 1. The obtained composite membrane for gas separation had a defect in the separation functional layer.

In the table, "average fiber diameter" means the average value of fiber diameter.

In the table, "diameter fluctuation" means fluctuation of the fiber diameter per 0.1 m in the longitudinal direction.

In the table, "periodic fluctuation" means periodic fluctuation of the fiber diameter in the longitudinal direction.

In the table, "period" means the period of periodic fluctuation of the fiber diameter in the longitudinal direction.

In the table, "structural period" means the structural period of a co-continuous structure.

Claims

Porous carbon fiber characterized in that the fiber diameter fluctuates in the longitudinal direction and the CV% calculated from the fluctuation of the fiber diameter is 1% or more and 10% or less.
The porous carbon fiber according to claim 1, wherein the average value of the fiber diameter is 20 μm or more and 5000 μm or less.
The porous carbon fiber according to claim 1 or 2, which has a co-continuous structure at least partially.
The porous carbon fiber according to claim 3, wherein the structural period of the co-continuous structure is 0.002 μm or more and 20 μm or less.
The porous carbon fiber according to any one of claims 1 to 4, wherein the fiber diameter varies from 3% to 10% in a section of 0.1 m in the longitudinal direction.
The porous carbon fiber according to any one of claims 1 to 5, wherein the fiber diameter varies periodically in the longitudinal direction.
The porous carbon fiber according to claim 6, wherein the period of the periodic fluctuation is 20 mm or more and 10000 mm or less.
A composite membrane for gas separation, which comprises the porous carbon fiber according to any one of claims 1 to 7 and a separation functional layer.
A module for a gas separation membrane containing the composite membrane for gas separation according to claim 8.